How many ltrs of water in a 4x4x2 tank?

The 4x2x2 is the external measurement, so we need to account for the glass.... your glass is probably about 3/4 of an inch thick. This is on 5 sides (not the top, obviously

This means your tank internally (it the empty space you can put water in) is about

3.875 x 2.875 x 2.933 feet

Which is about 14 cubic feet, or about 400 litres of water. Remember that your fish, ornaments, sand, filters etc all take up some of this space, so your 4x2x2 probably holds somewhere between 350 and 390 litres of water when full.